Communications networks are used for a wide variety of purposes, including communicating voice and data. There are many types of communications networks that are capable of communicating voice and data, including synchronous communications networks, which often use time division multiplexing (TDM) as a communications protocol to maintain timing synchronization at each circuit in the network, and asynchronous communications networks, which do not use a communications protocol that maintains timing synchronization. Performance Management (PM) tracking may be enabled by the nature of the communications network itself. In TDM communications networks, the base functionality of the synchronous “time division multiplexing” at a known rate provides measurable increments. In other words, the amount of data sent and received in a given time period is known by virtue of the rate of a TDM circuit and time period or window of the measurement. The time window between the ends of a circuit is synchronized by a time of day clock signal for data correlation of events in time. Each clock tick contains one and only one packet, and each packet may be checked for integrity so that over any time frame the errors may be recorded in any multiple of the timing clock desirable.
With non-synchronous packet systems, however, there are multiple system fundamental elements missing that impede accurate PM monitoring.
First, there is no end-to-end timing synchronization with Ethernet, although time-of-day synchronization may be provided by a network time protocol (NTP), which is the oldest time protocol operating on the Internet.
Second, packet sizes and transmission utilization (i.e., bandwidth use) may both vary and can be zero, while TDM bit rates are constant and, by definition, cannot be zero.
Third, packet switching and switching systems can be impacted by the over-subscription of traffic placed on them, whereas TDM systems are synchronized in a non-blocking way so as to not add transmission delay or drop data in nodes between physical media segments (e.g., fiber or copper). Furthermore, a packet communications system can experience variable data rates and lost packets as a normal system performance characteristic. Additionally, heavy over-subscription invokes QOS (Quality of Service) to manage congestion, which causes packet discard as a known characteristic.
Fourth, TDM packets contain no routing or switching information so they are passed through the TDM elements at line rate. In a packet based communications network, each packet may be evaluated by a routing or switching engine and transmitted at a packet matrix transmission rate. Because of the evaluation of each packet, line delay may be added during communication. The net effect of the added delay creates a situation where packets between the ends of the circuit are “in-flight” and do not arrive at known intervals. An analogous problem occurs in counting sporadic vehicles in traffic passing by two bridges that are located five miles apart. If, for example, a tally is made of the number of cars passing each bridge every 30 seconds, but without correlation as to which car the count started, there are typically some cars “in flight” between the bridges that are not counted at the second bridge. This same “in flight” problem exists with asynchronous networks and is compounded when considering distance and the fact that an Internet protocol (IP) network may be used to retrieve performance manager (PM) counters from the far ends of a circuit. Even if an element management system (EMS) or call control manager (CCM) that issues a request for a PM measurement to the ends of the circuit may be equidistant from each end, there may be no guarantee that the data time in flight may be the same on both ends of the network. This time difference may be nearly assured when the EMS system may be located much closer to one end of a circuit than it may be to the other and the measurement request experiences different transit times.
Fifth, service level agreement metrics are currently obtained by correlating the Ethernet or IP frames transmitted from one end of a customer circuit to a far end, which occurs in both directions as Ethernet and IP are uni-directional. Managing service level agreement metrics creates a load to a central processing unit (CPU) in the EMS system that may be correlating the transmitted and received packets from both ends of each circuit.
Accurate transmission performance management includes the ability to (i) track the amount of data received during a time window that correlates to the amount of data that the far end transmitted using performance counters, (ii) determine amount of data that the far end transmitted over a time window over which the data measurement occurred, and (iii) record those time windows into larger performance incremental counters. Managing these performance counters at a centralized system can be challenging given the number of network nodes and user network interfaces involved in a large scale network such as the Internet.